Wow, 2 of 13 tests from Graphics Diagnostic Disk
pass (presence detect, FIFO).
Ok, the PAL array already dumped does some modifications on bits before they are written back to the bit map memory (guess: ALU / plane select).
Now i am unsure about the 7220 clock. Is it really (E31) 31.188 Mhz / 4 = 7.797 Mhz ...?
A 31.188 quarz does not seem to exist in XTAL.H...
Datasheets says "7220 is available in 4.0 , 5.0, and 5.5 Mhz versions, 7220A in 6.0, 7.0, and 8.0 Mhz"
Pic: there is a plethora of good tests for different 7220 modes (complete with explanations and loop option...just press ALT for Help or L to loop after a failure)